patch 8.1.2315: not always using the right window when jumping to an error

Problem:    Not always using the right window when jumping to an error.
Solution:   Add the "uselast" flag in 'switchbuf'. (closes #1652)
diff --git a/runtime/doc/options.txt b/runtime/doc/options.txt
index ba96e83..b3d7579 100644
--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -7417,6 +7417,8 @@
 	   vsplit	Just like "split" but split vertically.
 	   newtab	Like "split", but open a new tab page.  Overrules
 			"split" when both are present.
+	   uselast	If included, jump to the previously used window when
+			jumping to errors with |quickfix| commands.
 
 						*'synmaxcol'* *'smc'*
 'synmaxcol' 'smc'	number	(default 3000)